Bright photographic lens system with rear stop diaphragm

ABSTRACT

A photographic lens system such as a telescopic lens system adapted for use with a rear stop diaphragm in which the backfocus is about 0.3F, the F number is about F/2.8, the system is small in size and aberrations are well compensated for. Front and rear lens groups are provided with the front group including in order from the object positive, negative, positive and positive lenses while the rear group includes negative and positive lenses. Preferred focal lengths, radii of curvature, refractive indices, and Abbe numbers for each of the lenses are disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lens system with a rear stop diaphragm.

2. Description of the Prior Art

In most ordinary photographic lenses, a stop diaphragm is provided between selected lenses within the lens system. It is, however, necessary to dispose a stop diaphragm behind the lenses, that is, in the body of the camera, in order to facilitate the operational construction of the camera mechanism. In such a lens system with a rear stop diaphragm, generally the aperture of a first lens group increases and it is therefore difficult to produce a lens system where various aberrations are well compensated for or corrected.

Ordinary telephoto lenses are so contructed that front lens groups thereof have total positive lens powers while rear lens groups thereof have total negative lens powers. In order to miniaturize such lens systems, in general, the respective powers of the front and rear lens groups are increased and in addition, the space between the front and rear lens groups is increased. That is, the physical length from the rear lens group to the focussed image surface or the "backfocus" is shortened to thereby reduce the overall length from a first surface of the front lens group to the image surface. However, in most lens systems having rear stop diaphragms it is considered that possible minimum backfocuses have already be attained. In this case, the increase of the power of each lens group is substantially the same as the decrease of the space between the front and rear lens groups. Moreover, the edge thickness of each positive lens is too small. For this reason, as a practical matter, the central thickness of the positive lens must be increased resulting in elongation of the front lens group. For a beam of light passing through the most peripheral portion of the first lens, a higher incident height is required for the front lens group. This leads to difficulties in miniaturizing the physical size of the lens system.

SUMMARY OF THE INVENTION

It is, therefore, the principal object of the present invention to provide a lens system with a rear stop diaphragm, specifically one in which the backfocus is about 0.3F, the F number is about F/2.8, the overall lens system is relatively small in size, and various aberrations are well compensated for over all the picture surface. Here, F represents the overall focal length.

For the lens system according to the present invention, the front lens group includes in order from the object, positive, negative, positive, and positive lenses and the rear lens group includes, in order, of negative and positive lenses. The lens system includes six lens elements grouped into six components and satisfying the following conditions.

(1) 0.45F<F₁.2.3.4 <0.6F

(2) -2.1F<F₂ <-1.7F

(3) -2.5F<r₂ <-1.5F, 0.8F<r₈ <1.5F, -F<r₂ +r₈ <-0.6F

(4) -1.5F<r₃ <-F, F<r₉ <1.5F, -0.2F<r₃ +r₉ <0.2F

(5) d₈ <0.1F

where:

F is the overall focal length,

F₁.2 . . . i is the focal length to the i-th lens,

F_(i) is the focal length of the i-th lens,

r_(j) is the radius of curvature of the j-th lens,

d_(j) is the lens thickness or the aerial space of the j-th lens,

ω is the half view angle,

n_(i) is the refractive index of the i-th lens at the d-line, and

υ_(i) is the Abbe number of the i-th lens.

BRIEF DESCRIPTION OF THE INVENTION

The specific nature of the invention, as well as other object, aspects, uses and advantages thereof, will clearly appear from the following description and from the accompanying drawings, in which:

FIG. 1 is a longitudinal view showing the lens system in accordance with the first embodiment of the invention;

FIGS. 2A to 2D show aberration curves obtained by the lens system in accordance with the first embodiment of the invention; and

FIGS. 3A to 3D show aberration curves obtained by the lens system in accordance with the second embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, as shown in FIG. 1, there is provided a lens system including a front lens group and a rear lens group, the front lens group including in order from the object a positive lens component L₁, a negative lens component L₂, a positive lens component L₃ and a positive lens component L₄. The rear lens group includes, in this order, a negative lens L₅ and positive lens L₆. The focal length according to the overall lens system may be varied by moving the lenses L₁ to L₄ in the front lens group.

The lens system according to the invention is characterized by the following five conditions as noted below, which will be described in greater detail later:

(1) 0.45F<F₁.2.3.4 <0.6F

(2) -2.1F<F₂ <-1.7F

(3) -2.5<r₂ <-1.5F, 0.8F<r₈ <1.5F -F<r₂ +r₈ <-0.6F

(4) -1.5F<r₃ <-F, F<r₉ <1.5F, -0.2F<r₃ +r₉ <0.2F

(5) d₈ <0.1F

wherein the terms in the inequalities are as defined previously.

In the lens system according to the invention, the telephoto ratio is about 1.0, the various aberrations are well corrected by the conditions (1) to (5), and the lens system may readily be miniaturized.

Condition (1)

    0.45F<F.sub.1.2.3.4 <0.6F

This condition is required for determining the focal length of the first lens group. If the focal length of the first lens group were smaller than the lower limit, the diameter of the first lens would increase resulting in an undesirable balance of the lens construction and at the same time making it impossible to provide good aberration compensation. If the focal length of the front lens group becomes larger than the upper limit, the overall length of the lens system is elongated.

Condition (2)

    -2.1F<F.sub.2 <-1.7F

This condition renders the second lens negative so that in the front lens group having as a whole a positive focal length, the chromatic aberration which may be deficient in compensation can be excessively compensated for. When the value of F₂ undesirably deviates from lower and upper limits of the range, the chromatic aberration is extremely deficiently or excessively compensated for.

Condition (3)

    -2.5F<r.sub.2 <-1.5F, 0.8F<r.sub.8 <1.5F, -F<r.sub.2 +r.sub.8 <-0.6F

In the lens system according to the present invention, a part of the power of the first lens group is loaded on the third lens L₃. However, a substantial part of the ability to compensate for the aberration depends on the first and fourth lenses L₁, L₄. The condition (3) is provided to share the aberrations with a good balance under the condition (1). The condition (3) is required to obtain good aberration compensation together with the following condition (4).

Condition (4)

    -1.5F<r.sub.3 <-F, F<r.sub.9 <1.5F, -0.2F<r.sub.3 +r.sub.9 <0.2F

With respect to the aberrations generated due to the condition (3), the second lens L₂ which is provided to compensate for the chromatic aberration, serves to largely compensate for aberration except for chromatic aberration. In this case, the condition (4) is required to balance the function of the second lens L₂ with the fifth lens L₅ disposed in the rear lens group so that the good aberration compensation can be totally achieved without excessive aberration loading on a specific lens. In addition, this makes it possible that in the focussing operation of this lens system, only the front lens group need be moved while the rear lens group remains stationary to thereby vary the overall focal length. However, if an excessive amount of aberration is concentrated on a specific lens in the front and rear lens groups, focussing by use of only the front lens group results in the possibility that the balance of the aberrations is destroyed which is of course undesirable. In such a focussing technique, the amount of movement of the lens is small in comparison with a focussing technique in which the focussing is carried out with the overall lens system. Since the variation of the light ray passing through the outermost portion of the lens is small, the diameter of the front lens in the front group moving focussing lens system can be maintained small. This is advantageous for miniaturization of the lens system.

Condition (5)

    d.sub.8 <0.1F

This condition also relates to the miniturization or compactness. When the value of d₈ increases more than the upper limit, the diameter of the front lens group must be great and at the same time the overall length must be increased.

Two examples of lens systems in accordance with the invention will now be given. The lens data in Examples 1 and 2 according to the present invention are given below for a lens system with a resultant focal length of F=100.

EXAMPLE 1

    ______________________________________                                          Aperture 1:2.8,                                                                          Overall Focal F = 100,                                                                         Viewing 2ω = 17°                       Ratio      Length          Angle                                               Radius of     Distance or                                                                               Refractive                                                                               Abbe                                        Curvature     Thickness  Index     Number                                      ______________________________________                                              r.sub.1 =   65.3546                                                                         d.sub.1 = 12.701                                                                          n.sub.1 = 1.5687                                                                       ν.sub.1 = 63.2                         L.sub.1                                                                             r.sub.2 = -180.5426                                                                         d.sub.2 =  2.303                                                  r.sub.3 = -156.0779                                                                         d.sub.3 =  5.778                                                                          n.sub.2 = 1.6034                                                                       ν.sub.2 = 38.0                         L.sub.2                                                                             r.sub.4 =  308.9377                                                                         d.sub.4 =  0.286                                                  r.sub.5 =   48.4589                                                                         d.sub.5 = 10.269                                                                          n.sub.3 = 1.5687                                                                       ν.sub.3 = 63.2                         L.sub.3                                                                             r.sub.6 =   74.0163                                                                         d.sub.6 =  0.286                                                  r.sub.7 =   38.5400                                                                         d.sub.7 =  8.310                                                                          n.sub.4 = 1.5163                                                                       ν.sub.4 = 64.1                         L.sub.4                                                                             r.sub.8 =   81.0961                                                                         d.sub.8 =  4.134                                                  r.sub.9 =  159.3317                                                                         d.sub.9 = 12.315                                                                          n.sub.5 = 1.7408                                                                       ν.sub.5 = 27.8                         L.sub.5                                                                             r.sub.10 =   21.6571                                                                        d.sub.10 = 14.832                                                 r.sub.11 =   51.8114                                                                        d.sub.11 = n.sub.6 = 1.6727                                                                       ν.sub.6 = 32.1                         L.sub.6                                                                             r.sub.12 = -385.0579                                                      ______________________________________                                    

EXAMPLE 2

    ______________________________________                                         Aperture Ratio 1 : 2.8, Overall Focal Length F = 100,                          Viewing Angle 2ω = 16°                                            Radius of     Distance or                                                                               Refractive                                                                               Abbe                                        Curvature     Thickness  Index     Number                                      ______________________________________                                              r.sub.1 = 93.2262                                                                           d.sub.1 = 9.300                                                                           n.sub.1 = 1.5687                                                                       ν.sub.1 = 63.2                         L.sub.1                                                                             r.sub.2 = -191.1157                                                                         d.sub.2 = 3.557                                                   r.sub.3 = -116.3420                                                                         d.sub.3 = 1.586                                                                           n.sub.2 = 1.7282                                                                       ν.sub.2 = 28.5                         L.sub.2                                                                             r.sub.4 = -1359.9035                                                                        d.sub.4 = 0.107                                                   r.sub.5 = 47.7816                                                                           d.sub.5 = 6.395                                                                           n.sub.3 = 1.5687                                                                       ν.sub.3 = 63.2                         L.sub.3                                                                             r.sub.6 = 93.2662                                                                           d.sub.6 = 0.133                                                   r.sub.7 = 27.6468                                                                           d.sub.7 = 11.352                                                                          n.sub.4 =  1.5163                                                                      ν.sub.4 = 64.1                         L.sub.4                                                                             r.sub.8 = 127.9306                                                                          d.sub.8 = 8.407                                                   r.sub.9 = 134.5698                                                                          d.sub.9 = 1.612                                                                           n.sub.5 = 1.6727                                                                       ν.sub.5 = 32.1                         L.sub.5                                                                             r.sub.10 = 15.9485                                                                          d.sub.10 = 18.014                                                 r.sub.11 = 35.3386                                                                          d.sub.11 = 2.691                                                                          n.sub.6 = 1.8052                                                                       ν.sub.6 = 25.4                         L.sub.6                                                                             r.sub.12 = 67.7565                                                        ______________________________________                                     

What is claimed is:
 1. A lens system comprising front and rear lens groups, said front lens group comprising, in order from the object, a first positive lens L₁, a second negative lens L₂, a third positive lens L₃ and a fourth positive lens L₄, and said rear lens group comprising, in order from the object, a fifth negative lens L₅ and a sixth positive lens L₆, said lenses being characterized by:

    ______________________________________                                          Aperture 1:2.8,                                                                          Overall Focal F = 100,                                                                         Viewing 2ω = 17°                       Ratio      Length          Angle                                               Radius of     Distance or                                                                               Refractive                                                                               Abbe                                        Curvature     Thickness  Index     Number                                      ______________________________________                                              r.sub.1 =   65.3546                                                                         d.sub.1 = 12.701                                                                          n.sub.1 = 1.5687                                                                       ν.sub.1 = 63.2                         L.sub.1                                                                             r.sub.2 = -180.5426                                                                         d.sub.2 =  2.303                                                  r.sub.3 = -156.0779                                                                         d.sub.3 =  5.778                                                                          n.sub.2 = 1.6034                                                                       ν.sub.2 = 38.0                         L.sub.2                                                                             r.sub.4 =  308.9377                                                                         d.sub.4 =  0.286                                                  r.sub.5 =   48.4589                                                                         d.sub.5 = 10.269                                                                          n.sub.3 = 1.5687                                                                       ν.sub.3 = 63.2                         L.sub.3                                                                             r.sub.6 =   74.0163                                                                         d.sub.6 =  0.286                                                  r.sub.7 =   38.5400                                                                         d.sub.7 =  8.310                                                                          n.sub.4 = 1.5163                                                                       ν.sub.4 = 64.1                         L.sub.4                                                                             r.sub.8 =   81.0961                                                                         d.sub.8 =  4.134                                                  r.sub.9 =  159.3317                                                                         d.sub.9 = 12.315                                                                          n.sub.5 = 1.7408                                                                       ν.sub.5 = 27.8                         L.sub.5                                                                             r.sub.10 =   21.6571                                                                        d.sub.10 = 14.832                                                 r.sub.11 =   51.8114                                                                        d.sub.11 = n.sub.6 = 1.6727                                                                       ν.sub.6 = 32.1                         L.sub.6                                                                             r.sub.12 = -385.0579                                                      ______________________________________                                    


2. A lens system comprising front and rear lens groups, said front lens group comprising, in order from the object, a first positive lens L₁, a second negative lens L₂, a third positive lens L₃ and a fourth positive lens L₄, and said rear lens group comprising, in order from the object, a fifth negative lens L₅ and a sixth positive lens L₆, said lenses being characterized by:

    ______________________________________                                         Aperture Ratio 1 : 2.8, Overall Focal Length F = 100,                          Viewing Angle 2ω = 16°                                            Radius of     Distance or                                                                               Refractive                                                                               Abbe                                        Curvature     Thickness  Index     Number                                      ______________________________________                                              r.sub.1 = 93.2262                                                                           d.sub.1 = 9.300                                                                           n.sub.1 = 1.5687                                                                       ν.sub.1 = 63.2                         L.sub.1                                                                             r.sub.2 = -191.1157                                                                         d.sub.2 = 3.557                                                   r.sub.3 = -116.3420                                                                         d.sub.3 = 1.586                                                                           n.sub.2 = 1.7282                                                                       ν.sub.2 = 28.5                         L.sub.2                                                                             r.sub.4 = -1359.9035                                                                        d.sub.4 = 0.107                                                   r.sub.5 = 47.7816                                                                           d.sub.5 = 6.395                                                                           n.sub.3 = 1.5687                                                                       ν.sub.3 = 63.2                         L.sub.3                                                                             r.sub.6 = 93.2662                                                                           d.sub.6 = 0.133                                                   r.sub.7 = 27.6468                                                                           d.sub.7 = 11.352                                                                          n.sub.4 =  1.5163                                                                      ν.sub.4 = 64.1                         L.sub.4                                                                             r.sub.8 = 127.9306                                                                          d.sub.8 = 8.407                                                   r.sub.9 = 134.5698                                                                          d.sub.9 = 1.612                                                                           n.sub.5 = 1.6727                                                                       ν.sub.5 = 32.1                         L.sub.5                                                                             r.sub.10 = 15.9485                                                                          d.sub.10 = 18.014                                                 r.sub.11 = 35.3386                                                                          d.sub.11 = 2.691                                                                          n.sub.6 = 1.8052                                                                       ν.sub.6 = 25.4                         L.sub.6                                                                             r.sub.12 = 67.7565                                                        ______________________________________                                    


3. A lens system according to claims 1 or 2, wherein said front lens group is moved for focussing operations. 